1. Field of the Invention
The present invention relates to a compound semiconductor device and, more particularly, a compound semiconductor device having a MESFET (Metal-Semiconductor FET).
2. Description of the Prior Art
In the power amplifier for the cellular phone base station, requests for the increase of the power supply voltage and the improvement of the power characteristic are increased in recent years, and thus the higher breakdown voltage is indispensable for the transistor employed in the power amplifier.
However, the MESFET having the GaAs channel layer is difficult to adapt to the increase of the power supply voltage since the GaAs channel layer is ready to be destroyed because of the electric field concentration, such MESFET is unsuitable for the power amplifier.
Therefore, the higher breakdown voltage of the MESFET is required.
In order to improve the breakdown voltage of the MESFET, as shown in FIG. 1, there is a structure in which the GaAs channel layer is replaced with the InGaP channel layer.
In FIG. 1, a buffer layer 102 made of AlGaAs is formed on a semiinsulating GaAs substrate 101, and then a channel layer 103 made of n-type In0.52Ga0.48P and a barrier layer 104 made of undoped AlGaAs are formed in sequence on the buffer layer 102.
Also, a contact layer 105 made of undoped GaAs is formed on the barrier layer 104. An opening 105a for exposing the barrier layer 104 is formed in the contact layer 105, and a gate electrode 106 is connected to the barrier layer 104 via the opening 105a. The gate electrode 106 and the barrier layer 104 are connected to each other to form Schottky junction.
In addition, a source electrode 107 and a drain electrode 108 are connected to the contact layer 105 on both sides of the gate electrode 106. The n-type impurity is ion-implanted at a high concentration into the contact layer 105 located under the source electrode 107 and the drain electrode 108. Accordingly, the source electrode 107 and the drain electrode 108 come into contact with the cap layer 105 to form ohmic-contact respectively.
The breakdown voltage of the above-mentioned MESFET having the InGaP channel layer is about 2 to 3 times higher than that of the MESFET having the GaAs channel layer.
As described above, since the MESFET whose channel layer is formed of InGaP has the high breakdown voltage rather than the prior art but the mobility in the channel layer 103 is low, the resistance is high and the mutual conductance (gm) is low. As a result, the gain is lowered rather than the prior art, and thus it is impossible to reap effectively the advantage such as the improvement of the breakdown voltage.
It is an object of the present invention to provide a compound semiconductor device having a transistor whose gain can be increased rather than the prior art.
The above subject can be overcome by forming the channel layer of the MESFET by InGaPSb.
Since the MESFET having the channel layer formed of InGaPSb make it possible to reduce the sheet resistance by using the small threshold voltage rather than the MESFET in the prior art, the mutual conductance and the gain of the MESFET can be increased without the reduction in the breakdown voltage.